This application claims priority to EP/01108480.3, filed Apr. 4, 2001 under the European Patent Convention and which is incorporated by reference herein in its entirety.
The invention relates to a method of producing a turbine blade in hollow section.
Gas turbines are used in many fields for driving generators or driven machines. In the process, the energy content of a fuel is used for producing a rotational movement of a turbine shaft. To this end, the fuel is burned in a combustion chamber, in the course of which air compressed by a compressor is supplied. In this case, the working medium which is produced in the combustion chamber by the combustion of the fuel and is under high pressure and high temperature is directed via a turbine unit connected downstream of the combustion chambers, where it expands to perform work. In the process, the impulse transfer, required for producing the rotational movement of the turbine shaft, from the working medium is achieved via turbine blades. To this end, a number of profiled moving blades are arranged on the turbine shaft, these moving blades, for directing the flow medium in the turbine unit, being complemented by guide blades connected to the turbine casing. In this arrangement, for suitable guidance of the flow medium, the turbine blades normally have a profiled blade body extended along a blade axis.
To achieve an especially favorable efficiency, such gas turbines, for thermodynamic reasons, are normally designed for especially high outlet temperatures of the working medium flowing out of the combustion chamber and into the turbine unit, these outlet temperatures ranging between about 1200xc2x0 C. and 1300xc2x0 C. At such high temperatures, the components of the gas turbine, in particular the turbine blades, are subjected to comparatively high thermal loads. In order to also ensure high reliability and a long service life of the respective components under such operating conditions, the components affected are normally designed to be coolable. In modern gas turbines, therefore, the turbine blades are normally designed as a xe2x80x9chollow sectionxe2x80x9d. To this end, the profiled blade body, in its inner region, has cavities (also designated as blade core) in which a cooling medium can be directed. Cooling-medium passages formed in such a way enable cooling medium to be admitted to the regions of the respective blade body which are especially subjected to thermal stress. In this case, an especially favorable cooling effect and thus especially high operating reliability can be achieved by the cooling-medium passages occupying a comparatively large spatial region in the interior of the respective blade body, and by the cooling medium being directed as close as possible to the respective surface exposed to the hot gas. On the other hand, in order to ensure sufficient mechanical stability and loading capacity in such a design, flow may occur in the turbine blade through a plurality of passages, in which case a plurality of cooling-medium passages to which cooling medium can be admitted and which are separated from one another in each case by comparatively thin dividing walls are provided.
Such turbine blades are normally produced by casting. To this end, a casting mold adapted in its contour to the desired blade profile is filled with blade material. To produce the aforesaid blade cores or flow passages for the cooling medium, xe2x80x9ccore elementsxe2x80x9d are arranged in the casting mold during the casting, these core elements being removed from the blade body after the casting operation has been effected, so that the cavities desired for the cooling-medium passages are produced. In this case, during the production of a turbine blade having a plurality of the cooling-medium passages separated from one another by dividing walls, a plurality of core elements adapted to the specific shape in each case are arranged in the casting mold. In order to hold these core elements in the correct position during the casting operation, on the one hand relative to one another and on the other hand relative to the casting mold, the core elements are normally connected to one another and/or to the casting mold via spacers. These spacers leave behind undesirable additional cavities when the core elements are removed, and these additional cavities impair the fluidic isolation, actually intended, of the respective core regions from one another and in particular from the outer region of the turbine blade. The spacers are therefore normally designed to be tapered in order to reliably rule out the formation of unacceptably large openings. In this case, the spacers are designed in such a way that, during the casting of the turbine blade, as far as possible a continuous surface or dividing wall which is not completely penetrated by the respective spacer is obtained at the respective location. Nonetheless, the cast turbine blade normally has weak points at the locations of the spacers, these weak points promoting at least local crack formation in the region in question. The defect or scrap rate during the production of the turbine blades is thus comparatively high.
The object of the invention is therefore to specify a method of producing a turbine blade in hollow section with which an especially low defect or scrap rate can be achieved.
This object is achieved according to the invention by a first core element being connected via a number of approximately cylindrical spacers to a further core element and/or to a casting mold, the cavities left in the casting mold by the core elements being filled by blade material, and the openings remaining in the turbine blade after the removal of the core elements and the spacers and produced by the spacers being closed by stopper elements.
In this case, the invention is based on the idea that a possible cause of defects during the production of the turbine blades can be seen precisely at those weak points which occur as a result of using tapered spacers when connecting the core elements. These weak points on the one hand impair the stability of the blade material at the location in question, but on the other hand can be identified only with difficulty, or cannot be identified at all, during a material test. Thus undiscovered weak points may remain in the material and may subsequently lead, due to crack formation at the location in question, to total failure of the turbine blade.
In order to effectively counteract this, cylindrical spacers are now used instead of conical or tapered spacers. Although these cylindrical spacers also leave behind weak points in the material of the cast turbine blade, these weak points can easily be discovered. While abandoning the principle of keeping the weak points small during the production of the turbine blades, provision is thus made, while tolerating comparatively larger weak points, for the latter to be made such that they can be discovered in an especially simple manner. The weak points, which can thus be reliably discovered, can then be closed effectively and in a manner which does not impair the subsequent operation of the turbine blade, by applying a closure element.
In this case, the spacers are preferably dimensioned in their longitudinal extent in such a way that their ends project beyond the blade profile produced, so that holes which pass completely through the respective structure are always produced during the casting of the turbine blade.
In order to ensure the tightness of the openings left by the spacers even during operation of the turbine blade under comparatively adverse operating conditions, the stopper elements, in an advantageous development, are upset, pressed, or otherwise manipulated after they have been inserted into the respective opening. Such pressing or upsetting ensures that the respective stopper element expands in its width in such a way that it forms an especially intimate positive-locking and frictional connection with the margin of the respective opening. The opening is thus closed in an especially effective manner.
To additionally secure the stopper element in its respective opening, it is advantageously brazed after it has been inserted into the respective opening.
The stopper element used may in each case be a suitable pin-shaped element. However, the stopper elements used are advantageously blind rivets or drive-in pins.
The advantages achieved with the invention consist in particular in the fact that, by deliberately tolerating comparatively large openings in the blade body cast to begin with, each weak point, caused by the spacers, in the blade body can be clearly identified. Concealed weak points are thus reliably avoided. In addition, by the subsequent insertion of the stopper elements, especially effective closure of the respective openings is ensured, so that the turbine blade can be loaded to a particular degree even under comparatively adverse operating conditions. In addition, the spacers may be dimensioned to be comparatively large, so that only a comparatively small number of spacers are required for reliable positioning of the core elements during the casting operation. Thus the number of openings or weak points produced overall is also reduced, so that the cost of closing these weak points again is kept especially low.